The field of this invention relates to vascular stents.
Vascular stents are structures that are designed to maintain the patency of a vessel in the body. The stent provides internal support to allow the circulation to proceed therethrough. Stents can be used in the vascular system in ureters, bile ducts, esophagus, and in many other tubular structures in the human body.
Stents can be tubular or can be made from wire. Stents are typically made from a metal or polymeric substance or a metal coated with polymers which are biocompatible or contain heparin to reduce blood clotting or other tissue reactions. Many prior designs have used a coil approach where a wire is helically wound on a mandrel. Yet other designs have evolvedxe2x80x94braided wire mesh and angulated wire forms wrapped on a spindle to form a coil.
U.S. Pat. No. 5,292,331 by Boneau and U.S. Pat. No. 5,403,341 describe such wire forms. These devices have very poor radial support to withstand the hoop strengths of the artery or vein and further are not suitable for arteries that are bent or curved or for long lesions; multiple stent are required. These designs do not provide any support to hold the wall of the artery, other than the memory of the metal.
Wall Stent, produced by Pfizer Inc., is a braided wire tube. Although this stent is flexible so as to be placed in curved arteries or veins and other body cavities, it does not have any radial strength imparted to it by design.
Wiktor, Pat. Nos. 4,649,922; 4,886,062; 4,969,458; and 5,133,732 describe a wire form stent. He describes stents made of wire helix made of a preformed wire which is in the sinusoidal form, in which either all or some of the adjacent strands are connected.
Arthus Fontaine, Pat. No. 5,370,683, also describes a similar device where a flat wire form of sinusoidal shape is wound on a mandrel to form a helical coil, the wire bends are xe2x80x9cUxe2x80x9d shaped and are connected to alternate xe2x80x9cUxe2x80x9d-shaped bands.
Allen Tower, U.S. Pat. Nos. 5,217,483 and 5,389,106 describes a similar device where the wire is preformed to a sinusoidal shape and subsequently wound on a mandrel to form a helical coil.
All of the above-described art fails to provide radial support. The preshaped wire form (sinusoidal in most of the prior art) is wrapped on a mandrel to form a coil. However, the forces imported by the vessel wall""s hoop strength are radially inward. In other words, the force is acting perpendicular to the plane of the U-shaped wire form. This means that the bends that are in the wire add no structural strength to the wire form to support the force produced by the wall, which is radially inward.
When we examine the simple coils, such as taught in U.S. Pat. Nos. Scott 5,383,928 or Gene Samson 5,370,691 or Rolando Gills 5,222,969, it is apparent that the spring coil will withstand substantial radial forces due to the vessel wall; however, all these stents are bulky in their pre-expanded form and are hard to place in small and curved arteries or veins of the body. Also, a major disadvantage of this design is that when the coil stent is placed in a curved artery or vein, it forms an xe2x80x9caccordionxe2x80x9d shape whereby some strands in the outer radius are spread and those of the inner radius are gathered. Spring coils can also xe2x80x9cflipxe2x80x9d to form a flat structure when a longitudinal force is applied on one side of the stent.
The other types of stents that have been developed are tube stents. Palmer, U.S. Pat. Nos. 4,733,665; 4,739,762; 7,776,337; and 4,793,348 describe such a tube stent of slotted metal tube. The slotted metal tube is expanded by a high-pressure balloon to implant the stent into the inside wall of the artery or vein.
Joseph Weinstein, U.S. Pat. No. 5,213,561 describes a similar stent made of tubular materials with slots cut into it. On expansion using a balloon, it forms a structure with diamond-shaped slots.
Henry Wall, U.S. Pat. No. 5,266,073 also describes a stent, tubular, that has slots machined into it. When expanded, the edges of the stent lock to form a cylinder. Not only is this device stiff and can only be used for short lesions, but also the diameter cannot be adjusted to meet the exact needs of the particular vessel but it is fixed to the predetermined sizes.
Lau and Hastigan, U.S. Pat. No. 5,344,426 describes a slotted tubular stent that has a structure similar to Henry Wall""s but has provided prongs that will lock in as the stent is expanded.
Michael Marin, U.S. Pat. No. 5,397,355 also describes a tubular slotted stent with locking prongs.
All the above-described tube stents, although typically providing substantial radial support when expanded, are not flexible enough to be placed in curved vessels. Arteries and veins in the human body are mostly curved and are tapered. As such, these tube stents suffer from this main disadvantage.
European patent document 042172982 employs wires that are doubled up and whose ends are snipped off to make a given joint. Such doubling up at the junction of two elements with snipped off free ends creates a potential puncture problem upon radial expansion. The sheer bulk of the doubled up wires makes them rotate radially outwardly away from the longitudinal centerline of the stent, while the plain ends on such an arrangement which are snipped off offer the potential of sharp points which can puncture or damage the intima. On the other hand, the apparatus of the present invention, employing sharp angles, as defined, avoids this problem in an embodiment which illustrates a continuous wire or wire-like member bent into a sharp angle. This type of structure alleviates the concerns of sharp edges, as well as the tendency of a doubled up heavy joint to rotate outwardly toward the intima upon radial expansion of the stem, as would be expected in the EPO reference 042172982.
Often these stents are layered with polymeric sheaths that are impregnated with biocompatible substances or can be coated with heparin or hydrogel. Most sheath-type coatings reduce endothelial cell growth through the stent, which is a major requirement in successful stenting of body cavities such as arteries and veins.
One of the problems with prior designs of slotted tube and wire stents is that in their expanded state, the openings in them become fairly large. This allows tissue to protrude through these openings or windows. When there are protrusions into the body cavity through the stent, it causes disturbances to the blood flow, causing activation of platelets causing blood clotting. This phenomenon can also enhance the process of restenosis due to the large area exposed for neointimal formation.
FIG. 1 depicts two rings of a stent of a design known in the prior art. Rings 10 and 12 are each sinusoidal, having respective peaks 14 and 16 joined together by crossties such as 18. Respective valleys 20 and 22 are deposed opposite each other to create a lengthy elongated opening 24, which has a length 26 nearly as long as the distance from opposing valleys 20 and 22. FIG. 2 illustrates what happens to the oblong openings 24 when the rings 10 and 12 are expanded radially to set the stent of the prior art shown in FIG. 1. As shown in FIG. 2, each of the openings 24 is quite large, allowing tissue growth to enter therethrough, as shown in FIG. 3, which shows more rings than the rings 10 and 12 illustrated in FIG. 1. The tissue growth 28 significantly constricts the blood flow passage through the stent of the prior art shown in FIGS. 1-3.
One of the objectives of the present invention is to provide a stent which overcomes this problem. Alternative solutions are illustrated to achieve the objective of making the opening smaller to provide better resistance to tissue growth into the blood flow passage through the stent. Thus, in one embodiment, the objective is accomplished by nesting adjacent rings which have sinusoidal bending so as to more closely pack them to reduce the opening sizes between them. In yet another embodiment, adjacent sinusoidal rings are made to be overlapping to again accomplish the objective of deceasing opening sizes in the expanded state for a stent of a given diameter and length. These and other objectives will be readily apparent to those skilled in the art from a description of the preferred embodiments of the invention below.
A flexible stent is disclosed which reduced openings between rings by two alternative techniques. In the first instance, adjacent sinusoidally bending rings are nested to compact them closer together to reduce the opening sizes therebetween. In another embodiment, adjacent sinusoidal rings overlap each other to achieve the same effect. The assembly can be covered with a graft as a support therefor.